Every autumn, Culex mosquitoes, the primary vector of West Nile virus in the United States, enter hibernation as daylight hours shorten. However, a new study has found that even backyard lights can delay these mosquitoes' sleep, giving them more opportunities to bite.
The study, recently published in the *Journal of Insect Physiology*, indicates that artificial light at night severely disrupts mosquitoes' ability to enter hibernation, potentially extending the disease season as cities become brighter. "This study is very compelling," said Dina Fonseca of Rutgers University.
As autumn arrives, mosquito larvae hatch into adults, actively storing energy and preparing to spend the winter in cool, dark places such as basements and caves. Scientists have long known that shortened daylight hours are a key signal for mosquitoes to enter hibernation, a state known as diapause.
Early laboratory studies have shown that low-intensity artificial light may interfere with mosquitoes and delay diapause. So, does the same phenomenon occur in the more complex environments of cities?
To find the answer, researchers placed small containers for raising mosquito larvae in the yards of residents in Columbus, Ohio. Some were placed directly under existing outdoor lights, while others were hidden in naturally dark corners of the same yards. After the larvae developed into adults, the researchers collected these containers to test whether the mosquitoes inside had entered diapause or were still in their active, blood-feeding and reproductive phase.
The study showed that in September, the proportion of mosquitoes raised under lights that entered diapause was about one-quarter that of mosquitoes in the dark. By October, the contrast became even more pronounced: every mosquito in the dark entered diapause, while 59% of the mosquitoes exposed to light remained active.
"Light pollution has a much stronger inhibitory effect on hibernation than temperature," said Lydia Fyie of the University of Maine, the paper's lead author. Even light levels of only 0.87 lux (roughly equivalent to starlight at night) were enough to trigger mosquito activity. If mosquitoes remain active for longer periods, they will have more opportunities to contract and transmit diseases. This also means that more mosquitoes may breed before winter, leading to a larger mosquito population the following spring and a larger population throughout the summer.
However, Fonseca points out a key limitation of the study: it did not use wild-caught mosquitoes, but rather a laboratory-bred Culex mosquito population, which has been bred for multiple generations under artificial conditions and may exhibit different responses. She acknowledges, however, that wild Culex mosquitoes are very uncooperative in artificial environments, making any research on them in the laboratory extremely difficult.
Researchers say the next step will be to conduct long-term seasonal monitoring of wild mosquito populations in high-light and low-light environments, tracking the changes in the onset and end of their diapause over many years.
“There is mounting evidence that nighttime light has a significant impact on the biology and behavior of mosquitoes,” says Katie Westby of Washington University in St. Louis. “How many mosquitoes will extend their active period, and what this means for their overwintering, remains an open question.”
